1. Field of the Invention
The present invention relates broadly to the treatment and recycle of rubber products and still more particularly to the use of recycled crumb rubber.
2. Discussion of Background
Existing efforts to recycle used rubber, in particular used tires, into new rubber articles, especially tires, have met with only limited commercial success. In the United States alone, there are currently billions of tires stock-piled in long term storage with additional millions being added annually to such stock piles. Because of the large volume involved with tires, this discussion will be directed to tires although much of these comments are applicable to other new and used rubber products.
Because used rubber is usually processed in the form of crumb rubber, references herein will be to rubber in that form. However, crumb rubber is merely one example of a used rubber product suitable for processing as described herein. An existing limitation in the recycling of used tire material is that the used tire rubber can not be readily mixed in economical proportions to form suitable new tire polymer mixes having acceptable cured properties.
During the vulcanization process of new tires, chemical accelerators, promoters, and/or initiators, are used and large numbers of sulfur cross-links are produced in the vulcanized tire rubber. It is generally believed in the art that the sulfur compounds which are present in used tire rubber render the previously vulcanized rubber unsuitable for incorporation in significant quantities into a new rubber formulation. Reformulation of used tire rubber particles with new polymer materials results in a brittle compound unsuitable for many uses such as automobile or truck tires. Heretofore, many efforts to reclaim scrap rubber have included a physical sheering process which is suitable for a rubber which can be mixed with asphalt, forming asphalt rubber. Such use is taught in U.S. Pat. No. 5,304,576.
It is also known to take used rubber and depolymerize the vulcanized rubber in an organic solvent and then recover various polymerized fractions as taught in U.S. Pat. No. 5,438,078. Similarly, U.S. Pat. No. 5,264,640 teaches taking scrap rubber from used tires and regenerating the monomeric chemicals which are subsequently recovered. This method uses gaseous ozone to break down the crosslinked structure of the rubber followed by thermal depolymerization in a reaction chamber. U.S. Pat. No. 5,369,215 teaches a similar process in which used tire material may be depolymerized under elevated temperatures and at a reduced pressure to recover the monomeric compounds.
U.S. Pat. No. 4,104,205 teaches a microwave method to devulcanize rubber from hose end trim and butyl tire bladders. While tire tread material was also treated, difficulties in exothermic reactions and physical properties of the microwaved materials were noted.
U.S. Pat. No. 5,891,926, incorporated herein by reference, uses elevated temperatures, pressure, and extracting solvent to recover a vulcanized polymer from cured rubber.
U.S. Pat. No. 4,341,667 teaches that the green strength of elastomers reclaimed through heat, microwave, chemical treatments, or physical shearing can be improved by the addition of butene polymers.
U.S. Pat. No. 4,469,817 teaches the microwave treatment of a vulcanized rubber. The treatment uses microwaves to raise the temperature of the rubber to a devulcanization temperature followed by rapid cooling with water.
U.S. Pat. No. 4,665,101 is an improvement to conventional heating of used rubber where microwave heating is additionally used. The combination of the heating methods is used to provide a more uniform heating profile and avoid extremes of localized temperatures within the treated material.
U.S. Pat. No. 5,275,948, incorporated herein by reference, teaches the use of chemolithotrophic microorganisms to remove sulfur from the surface of finely ground scrap rubber. Preferred organisms include Thiobacillus species which oxidize elemental sulfur to sulfuric acid and which is released into the suspension culture.
U.S. Pat. No. 5,597,851, incorporated herein by reference, teaches the use of microorganisms to desulfurize finely ground rubber particles. Thiobacillus sp. and Sulfolobus acidocaldarius are used to bioprocess rubber particles for at least 24 hours but prior to complete oxidation of surface sulfur.
The present application relates to commonly assigned U.S. application having Ser. No. 09/542,744 filed Apr. 4, 2000 entitled xe2x80x9cMicrobial Processing of Used Rubberxe2x80x9d and having Attorney Docket No. WSR-15, and which is incorporated herein by reference. The present application also relates to commonly assigned U.S. patent application Ser. No. 09/542,201 filed Apr. 4, 2000 entitled xe2x80x9cMicrowave Treatment of Vulcanized Rubberxe2x80x9d having Attorney Docket No. WSR-13, and which is incorporated herein by reference. Additional teachings of a surface treatment protocol for the treatment of crumb rubber may be found in the commonly assigned U.S. application bearing U.S. Ser. No. 08/853,130 entitled xe2x80x9cNonaqueous Ozonation of Vulcanized Rubberxe2x80x9d, now U.S. Pat. No. 5,998,490, and which is incorporated in its entirety herein by reference.
There remains a strong need for a practical, economical system for processing used rubber into a material which can be incorporated at a substantial loading level into new rubber compounding mixtures. There is room for improvement with respect to the existing use and applications of used rubber and to the treatment of used, vulcanized rubber to render the rubber more suitable for high performance products such as tires.
The present invention is a process and the resulting product of the process in which previously vulcanized rubber may be incorporated into polymer mixes for new rubber products, including tires, at much greater levels than used heretofore. The present invention provides a process and a resulting product of the process wherein previously vulcanized, used crumb rubber has its bulk and surface chemistry altered. The chemical alteration employs separate treatment stages which, in one example, includes the use of bacteria as a biological agent to oxidize the surface of the crumb rubber, along with an additional separate surface and bulk treatment using microwave radiation.
When compared to untreated crumb rubber, or to crumb rubber treated solely with microwave or with biological agents, the combination of the biological and microwave treated crumb rubber has generally improved properties useful for incorporation into new tire or virgin rubber polymer formulations. The improved properties for the composite polymer formulations include plasticity, tensile strength, elongation @ break, and energy @ break. The properties listed above are generally better than the combination of untreated crumb and new tire rubber mix control values when the combination of treated crumb rubber and new tire rubber was analyzed and evaluated in Banbury tests. For some properties, the treated crumb rubber/new rubber mix exceeds a benchmark value seen in a 100 percent new rubber compound.
In accordance with this invention, it has been demonstrated that particles of used crumb rubber can be reacted with active cultures of a bacillus-like bacterium isolated from a natural hot sulfur spring. The preferred isolate shows most favorable growth at 65 degrees C. and has been found to react with Sxe2x80x94S, and Sxe2x80x94C bonds and provides a more reactive site on the rubber surface. In one embodiment, the bacterium interacts with the surface sulfur constituents and alters the surface chemistry and reactivity of the so treated vulcanized crumb tire rubber. Following the biotreatment, additional treatment protocols using microwave energy are applied which further enhances the end qualities of the crumb rubber particle.
The combination of biological and microwave treatment (hereinafter xe2x80x9ctandem treatmentsxe2x80x9d) of used rubber particles changes the bulk and surface properties of the crumb rubber particles to an extent that the rubber particles can be integrated into a new tire polymer mix at much higher levels than previously thought feasible. The tandem treatment of the crumb rubber provides a treated rubber product which is compatible with the new rubber polymer component of a tire mix.
The use of the tandem treatment to modify select chemical species present on the surface of the crumb rubber provides a process which can be carried out on an economical basis. The tandem process is thought to be particularly useful in that the combination of separate treatment steps is believed to alter a wider variety of chemical tire/rubber additives than either treatment protocol used as the sole treatment. The ability to alter by surface and bulk treatments the chemical constituents, further enhances the usefulness and compatibility of the treated used rubber with new or virgin polymer. The tandem treatments also provide a more standardized crumb rubber for use in recycling operations, including the incorporation of significant quantities and appreciable sizes of treated crumb rubber into polymer formulations for automobile and truck tires.
The use of the tandem treatments provides the potential of yielding a process and resulting product which can be obtained on an economical basis at a commercial scale and in an environmentally acceptable manner. Further, the elevated temperatures at which the preferred microorganisms operate can be maintained using waste heat which results from any of several steps in the commercial manufacturing of new tires. When the microwave treatment is the final step of the tandem process, the elevated temperatures generated in the crumb rubber help provide a more uniform crumb rubber feed stock. Further, the microwave step also dries the crumb rubber which facilitates the use and handling of the crumb rubber in preparing polymer mixes.
It is thus an object of this invention to provide a tandem process which increases by an effective amount the reactivity of used crumb rubber such that an increased amount of used crumb rubber can be mixed with a new tire polymer mix.
It is another object of this invention to provide a crumb rubber having an improved surface chemistry suitable for use in compounding a new tire rubber mix.
It is still another object of this invention to provide a treated crumb rubber product having a consistent surface chemistry and bulk properties in which polymer degrading constituents have been altered.
It is yet another object of this invention to provide a compounded rubber mix containing large sizes of treated rubber particles.
It is yet another object of this invention to provide a compounded rubber mix containing increased amounts of crumb rubber in the formulation.
It is a further object of this invention to provide a process of devulcanizing a rubber particulate in which the devulcanization is substantially limited to targeted chemical species and bonds which yields beneficial properties in recycled rubber products.
These and other objects of this invention are provided by a process for devulcanizing and/or otherwise conditioning or modifying particulate rubber comprising: providing a supply of crumb rubber, said crumb rubber having a plurality of Cxe2x80x94S and Sxe2x80x94S bonds; exposing the crumb rubber to microorganisms capable of using surface constituents as metabolic and/or energy sources, thereby oxidizing a first portion of the Cxe2x80x94S and Sxe2x80x94S bonds; and, exposing further the scrap rubber to microwave radiation and radiant heat, thereby oxidizing a second portion of Cxe2x80x94S and Sxe2x80x94S bonds; thereby, providing a crumb rubber having a treated surface and improved bulk properties facilitating the formulation and curing of the particulate rubber into a new rubber product.